1. Field of the Invention
The invention relates to a semiconductor device and a method of fabricating the same.
2. Description of the Related Art
In these days, a semiconductor memory device has been required to operate at a higher rate, have a greater capacity for storing data therein, and operate in less power consumption. To such requirements, there has been suggested a dynamic random access memory (DRAM), which is much in demand now, having a capacitive or insulating film of a capacity device or a memory node which film is composed of metal oxides having a greater dielectric constant than that of a silicon dioxide film in order to make it possible to form memory cells in a circuit smaller. Some of such metal oxides are ferroelectric. There has been also suggested a non-volatile memory utilizing ferroelectric characteristics of those metal oxides.
Japanese Unexamined Patent Publication No. 7-38068 published on Feb. 7, 1995 has suggested a semiconductor memory device having a capacitive film composed of high-dielectric substance. FIG. 1 illustrates the suggested semiconductor memory device.
LOCOS oxide films 803 are formed at a surface of a silicon substrate 801 to thereby define a device formation region therein. Below LOCOS films 803 are formed channel stopper regions 804. Source/drain regions 802 are formed at a surface of the silicon substrate 801. Gate electrodes 805 acting as word lines are formed on LOCOS oxide films. A signal line 806 acting as a bit line is formed on the silicon substrate 801 between the source/drain regions 802.
An interlayer insulating film consisting of a silicon dioxide film 807 and a silicon nitride film 808 is formed on the silicon substrate 801. The interlayer insulating film is formed therethrough with contact holes 809 reaching the source/drain regions 802. Each of the contact holes 809 is filled with a plug 810 composed of an electrically conductive material such as metal.
A capacity device having a capacitive film composed of high-dielectric substance is formed on the interlayer insulating film. The capacity device is in electrical connection with the source/drain regions 802 through the plug 810. The silicon nitride film 808 and the plug 810 are both planarized at the same level.
The capacity device is comprised of a plurality of lower or storage electrodes 811, a capacitive insulating film 812 entirely covering the lower electrodes 811 therewith, and an upper or plate electrode 813 deposited all over the capacitive insulating film 812.
Over the upper electrode 813 are formed a metal wiring layer (not illustrated), and an interlayer insulating film 814 for electrically insulating the metal wiring layer and the upper electrode 813 with each other, to thereby constitute a semiconductor memory device.
After the metal wiring layer has been formed, the product is thermally annealed in hydrogen atmosphere in order to reduce a dispersion in a threshold voltage of transistors arranged on a surface of the silicon substrate, and a dispersion in a current for driving transistors. This thermal annealing compensates for defects, such as a trap level, formed at an interface between the capacity device and the interlayer insulating film making contact with the capacity device.
However, the above-mentioned semiconductor memory device illustrated in FIG. 1 has shortcomings as follows.
First, since the thermal annealing in hydrogen atmosphere is carried out after the capacity device has been formed, the capacitive film composed of metal oxide is unpreferably reduced with the result of deterioration of capacitive characteristics of the capacity device.
Secondly, transistors formed on layers located below a layer on which the capacity device is formed may be deteriorated with respect to performance and reliability thereof. Specifically, as illustrated in FIG. 1, since the silicon nitride film 808 entirely covers the silicon dioxide film 807 except the plugs 810, if the product is thermally annealed in hydrogen atmosphere after the capacity device has been formed, the silicon nitride film 808 acts as a barrier for hydrogen to reach layers on which transistors are formed. As a result, performance of transistors and reliability for transistors are deteriorated, and a characteristic of transistors is not uniformized.